1. Field of the Invention
The present invention relates to a pulse transformer and, more particularly, to a surface-mount pulse transformer using a drum-type core.
2. Description of Related Art
In recent years, in a circuit component such as a connecter, a pulse transformer is widely used for isolating a differential signal at an input side (primary side) and a differential signal at an output side (secondary side). In order to mount a plurality of pulse transformers on a printed circuit board at high density, it is preferable to use a surface-mount pulse transformer using a drum core (see Japanese Patent Application Laid-Open Nos. 2009-302321 and 2010-109267).
A pulse transformer described in the Japanese Patent Application Laid-Open No. 2010-109267 has a configuration in which primary-side terminal electrodes and a secondary-side center tap are formed in one flange of a drum core, and secondary-side terminal electrodes and a primary-side center tap are formed in the other flange. Wires constituting a pair of coils are wound around a winding core of the drum core, and each end portion of the wires is connected to a corresponding terminal electrode or center tap. Such a pulse transformer is required to have a configuration capable of accurately winding the wires using an automatic winding machine and reliably connecting each end of the wires to the corresponding one of the terminal electrodes.
FIG. 13 is an exemplary plan view illustrating a terminal electrode structure of a conventional pulse transformer.
A pulse transformer illustrated in FIG. 13 has a drum core. A pair of primary-side terminal electrodes 6a and 6b and a secondary-side center tap 6c are provided on one flange 4A of the drum core, and a pair of secondary-side terminal electrodes 6d, 6e and a primary-side center tap 6f are provided on the other flange 4B of the drum core. In the flange 4A, the primary-side terminal electrode 6b is distanced from the secondary-side center tap 6c so as to ensure withstand voltage between the primary and secondary sides. Similarly, in the flange 4B, the secondary-side terminal electrode 6e is distanced from the primary-side center tap 6f. 
A pair of coils constituting the pulse transformer includes four wires S1, S2, S3, and S4. The wires S1 and S2 constitute a primary-side coil, and wires S3 and S4 constitute a secondary-side coil. In FIG. 13, only lead sections of the wires are illustrated, and winding portions thereof are omitted. An end S1a of the wire S1 and an end S3b of the wire S3 are led out from the winding core 3 and connected to the corresponding terminal electrodes 6a and 6c, respectively, without crossing the other wires. On the other hand, an end S2b of the wire S2 and an end S4a of the wire S4 are each led out from a side surface side of the winding core 3 which is a far side from the terminal electrode to which each of the wires S2 and S4 should be connected so as to cross a center axis Y0 of the winding core 3 and connected to the corresponding terminal electrode. Specifically, the wire S2 is led out from a left side surface 3b of the winding core 3 so as to cross the center axis Y0 and connected to the terminal electrode 6b. The wire S3 is led out from a right side surface 3a of the winding core 3 so as to cross the center axis Y0 and connected to the terminal electrode 6c. Thus, wires S2 and S3 are connected to the corresponding terminal electrodes 6b and 6c, respectively, after crossing each other.
However, the above-described conventional pulse transformer has a problem in that the lead section of one wire that crosses the other may also contact a different terminal electrode from its connection target terminal electrode. That is, the wire S4 that crosses the wire S2 at the flange 4A side may contact the terminal electrode 6b in the course of connecting the end S4a to the terminal electrode 6c, and the wire S1 that crosses the wire S3 at the flange 4B side may contact the terminal electrode 6e in the course of connecting the end S1a to the terminal electrode 6f. To avoid such contact, it is necessary to bend the end portion sharply for connection to the corresponding terminal electrode, making it very difficult to handle the wires with an automatic winding machine.